1. Field of the Invention
The present invention relates to a spinel (AB2O4) type lithium manganese complex oxide which is used as a cathode active material in a lithium secondary battery, and also relates to a lithium battery formed by using the complex oxide.
2. Description of the Related Art
In recent years, there has been a trend to making electronic devices portable and cordless. As an electric power source for use in the electronic devices, there has been a strong demand for a secondary battery which is compact in size, light in weight and has a high energy density. There has been in practical use a non-aqueous electrolytic solution type lithium secondary battery as a secondary battery satisfying the above requirements.
As the cathode active material for use in the non-aqueous electrolytic solution type lithium secondary battery, some lithium-containing complex oxides such as lithium nickelate (LiNiO2), lithium cobaltate (LiCoO2), and lithium manganate (LiMn2O4) have been proposed, with the lithium cobaltate (LiCoO2), and lithium manganate (LiMn2O4) already in practical use.
With a lithium secondary battery whose cathode active material is formed by a lithium manganate (LiMn2O4) coming from a manganese resource, which is more abundant than cobalt resource, one can use a cathode active material represented by Li(Mn2xe2x88x92xLix)O4 in which the part of the manganese site has been replaced by lithium or a cathode active material represented by Li1+xMn2xe2x88x92xO4 in which the atom ratio of lithium and manganese is within a predetermined range. As a result, it is possible to obtain a secondary battery which has a large initial electric discharge capacity and has an excellent cycle property ensuring that there would be no deterioration in discharge capacity even when the electric charge and discharge are repeated (Japanese Unexamined Patent Publication No. 7-282798 and Japanese Unexamined Patent Publication No. 9-241024).
However, with the above lithium secondary battery formed by using the above cathode active materials, although it has not been found that there is a considerable deterioration in its capacity when conducting a cycle of electric charge and discharge under conditions close to room temperature, if the cycle of electric charge and discharge is carried out at a temperature higher than 60xc2x0 C., it has been found that the charge and discharge capacities are deteriorated to an extreme extent.
Further, as shown in the following equation (1) which may be used to calculate a theoretical capacity of a 4V area of a spinel type lithium manganate, with the lithium manganate represented by Li(Mn2xe2x88x92xLix)O4, a larger substitution amount x of substituting lithium for manganese, will cause the charge and discharge capacities to be reduced to a larger extent. For example, if the substitution amount x is 0.33, the lithium manganate will be Li(Mn1.67Li0.33)O4=Li1.33Mn1.67O4=Li4Mn5O12. At this moment, since an average valence of manganese becomes 4.0 and it is not possible at all to conduct an electric charge, its capacity will become 0 mAh/g.
xe2x80x83Theoretic Capacity=(1xe2x88x923x)xc2x7(A x E/3.6)/Mxe2x80x83xe2x80x83(1)
x: substitution amount for substituting lithium for manganese
A: Avogadro number
E: elementary electric charge
M: molecular weight of Li(Mn2xe2x88x92xLix)O4.
However, the inventors of the present invention found that a lithium manganate in which the manganese site has been displaced by lithium can produce a larger capacity than the above theoretic capacity calculated by the above equation.
The inventors of the present invention conducted a determination of mole ratio of lithium and manganese, and also carried out a titration of manganese, thereby correctly calculating mole ratios of lithium, manganese and oxygen. As a result, it was understood that the lithium manganate represented by Li(Mn2xe2x88x92xLix)O4 has a composition of Li(Mn2xe2x88x92xLix)O4xe2x88x92xcex4, i.e., actually has an oxygen defect or deficiency.
A lithium manganate having an oxygen defect, Li(Mn2xe2x88x92xLix)O4xe2x88x92xcex4 (0xe2x89xa6xxe2x89xa60.02, xe2x88x920.015xe2x89xa6xcex4xe2x89xa60.012) has been disclosed (Japanese Unexamined Patent Publication No. 10-21914). In the conventional solid phase method for synthesizing a lithium manganate in which the manganese site has been displaced by lithium, it has been shown that the lithium manganate was generated having a composition of Li(Mn2xe2x88x92xLix)O4xe2x88x92xcex4 and having an oxygen defect, and that a larger amount of oxygen defect will cause a larger deterioration in the charge/discharge cycle property.
A lithium manganate having a predetermined range of oxygen defect has an excellent cycle property when the battery is charged or discharged at a temperature close to room temperature, but it has been found that the cycle property will be deteriorated rapidly if an electric charge or an electric discharge is conducted at a temperature (60xc2x0 C.) which is higher than a room temperature.
The inventors of the present invention have noted the oxygen defect amounts before and after the deterioration of cycle property in charge and discharge, conducted an active and sufficient research, and finally found that such kind of lithium manganate will increase the oxygen defect amount when it is at a high temperature.
With the above lithium manganate where the manganese has an average valence of 3.505, it was found that at room temperature there was no deterioration in the charge and discharge cycle property, which deterioration will otherwise occur because of the Jahn-Teller effect. However, at a high temperature, the cathode active material will be chemically reduced so that an oxygen defect will occur. In order to compensate for electric charges, Mn4+ is reduced to Mn3+ and the average valence of manganese will become smaller than 3.5. Once the average valence becomes smaller than 3.5, the Jahn-Teller distortion will cause a phase transition in which a cubic system is changed into a tetragonal system, hence causing a considerable change in the volume thereof, resulting in a deterioration in the charge and discharge cycle property.
It is an object of the present invention to provide a spinel type lithium manganese complex oxide for use as a cathode active material in a lithium secondary battery, capable of solving the above problem associated with the conventional spinel type lithium manganese complex oxide, enabling the manganese to have an average valence of 3.5 or more, having a large electric charge and discharge capacity, having an excellent charge and discharge cycle property, in particular at a high temperature, and also to provide a lithium secondary battery having the above properties.
The spinel type lithium manganese complex oxide consists essentially of Li, Mn and O, and is represented by a general formula LixMn2Oy, where x satisfies xxe2x89xa71.04 and y is in the ranges satisfying the following inequarities.
yxe2x89xa70.667x+3.333
yxe2x89xa61.333x+2.667
y less than 0.333x+3.783
It is preferable that y is in the ranges satisfying the following inequarities.
yxe2x89xa7x+3
yxe2x89xa61.333xc3x97x+2.667
y less than 0.333xc3x97x+3.783
More preferably, y is in the ranges satisfying the following inequarities.
y greater than 1.333xc3x97x+2.655
yxe2x89xa61.333xc3x97x+2.667
y less than 0.333xc3x97x+3.783
According to the present invention, when the lithium amount and oxygen amount of the spinel type lithium manganese complex oxide can satisfy the above various requirements, it is possible to obtain the following effects. Namely, one is able to obtain a large discharge capacity in the cathode active material of the lithium secondary battery. Further, even when the reduction resistance of the material is increased and charge and discharge are carried out at a high temperature, the occurrence of an oxygen defect can be inhibited, and thus it is not necessary to conduct a process of electric charge compensation which is otherwise necessitated by the oxygen defect due to manganese. For this reason, it is possible to maintain the average valence of manganese at a value (3.5 or above) which does not cause the Jahn-Teller distortion, and thus considerably prohibit a large volume change which possibly occurs due to a phase transfer caused by the Jahn-Teller distortion. Therefore, it is possible to obtain a lithium manganese complex oxide having an excellent cycle property of charge and discharge.
By using the above lithium manganese complex oxide as a cathode active material, one can obtain a lithium secondary battery having the above excellent properties.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.